1. Field of the Invention:
The present invention relates to a windshield glass for a vewhicle which comprises an antenna element for receiving a radio or TV signal.
2. Description of the Prior Art:
Conventionally, a rear windshield glass provided with a plurality of heating conductive wires for defogging and an antenna wire for receiving radio or TV signals is known. Although such a glass antenna for a vehicle can relatively sensitively receive medium frequency waves, it is insufficient reception sensitivity (gain) with respect to ultrashort waves such as FM waves and has a poor S/N ratio of a reception signal.
For example, glass antennas having conductive patterns shown in FIGS. 1A and 1B are conventionally used. As shown in FIGS. 1A and 1B, a heating conductive wire group 2 for defogging is provided on a rear windshield glass 1 of a vehicle, and a heating current is supplied thereto through buses 3 and 4. An antenna wire 5 for receiving very high frequency such as FM waves is provided above the conductive wire group 2 parallel thereto, and a reception signal is derived from a feeding point 6 provided substantially at the center of the wire 5.
The heating conductive wire group 2 is also used as an antenna for the medium frequency band. For this purpose, the uppermost wire of the heating conductive wire group 2 is connected to the antenna wire 5.
The antenna wire 5 shown in FIG. 1A is a single element type, and one end of a single antenna element 5a extending in the horizontal direction is connected to the heating conductive wire group 2 through a coupling wire 5b. FIG. 1B shows a modification of the antenna pattern shown in FIG. 1A, in which in order to enhance a reception gain, a part 5a' of the element 5a extends along the uppermost stage of the heating conductive wire group 2.
The antenna patterns shown in FIGS. 1A and 1B are of center feeding point type. The pattern shown in FIG. 1B, for example, provides a relatively high sensitivity, as shown in a reception level graph of FIG. 2. The ordinate of FIG. 2 indicates an average reception level AVE within the entire azimuth range of the antenna.
However, in the case of a center feeding point type antenna, a feeder line in a vehicle must be extended to an upper central portion of the windshield glass 1. It causes a problem in mounting the feeder line.
As shown in FIGS. 3A to 3C, another type of antenna pattern in which the feeding point 6 is shifted to a side portion of the windshield glass 1 through a lead wire 5c is proposed. In this type, as shown in FIG. 3A, when the feeding point 6 is simply provided at the side portion, the effective length of the antenna element is changed due to the lead wire 5c and sensitivity is lowered, as shown in a reception level graph shown in FIG. 4. Variations in frequency characteristics, however, are suppressed as compared to that of FIG. 2 and the reception level is stabilized.
In order to improve sensitivity of the antenna of FIG. 3A, the lead wire 5c is provided near along a glass edge from the center of the antenna element 5a to the feeding point 6. Thus, the lead wire 5c is AC coupled to a body (ground potential) of a vehicle, and reception characteristic is compensated, thus reducing an influence of the lead wire 5c on sensitivity. However, the lead wire 5c is concealed in a weatherproof strip of the glass edge, and may be disconnected due to electrical corrosion by water over a long period of time.
Referring to FIG. 3B, a multifolded wire 5b is used for adjusting the length of the element, which corrects the reception characteristics. In this case, however, in order to obtain a required length of the element, a distance between each two adjacent folded portions of the multifolded wire 5d becomes narrow, and AC coupling occurs therebetween. Thus, an expected effect in adjustment of the length cannot be obtained.
Referring to FIG. 3C, an earth element 7 is provided along the lead wire 5c so as to correct the characteristics, and an influence of the lead wire 5c applied to the antenna characteristics is reduced. However, an earth terminal 8 is additionally required, resulting in an increased cost.